1. Field of the Invention
The present invention relates to a power supply device for converting the power generated in a fuel cell to a particular voltage level and supplying it to a load.
2. Description of the Prior Art
Recently, secondary batteries such as lithium ion battery and nickel metal-hydride battery have been used as the power source for portable electronic devices such as personal computer and electrical machines such as electric tool. However, when such a device is operated by using a secondary battery, the continuous operational period of the device is limited, because of restriction in battery capacity. For example, when portable personal computer is operated with a secondary battery, the continuous power-supply period is usually, for example, about 4 hours.
Recently on the other hand, fuel cells that can supply power continuously for a longer period are attracting attention. In the case of power supply to personal computer, a fuel cell supplying power continuously for 20 to 40 hours is desirable.
The fuel cell has a configuration in which multiple unit cells each having an electrolyte layer held between a fuel electrode (−) and an air electrode (+) are laminated, and the power is generated in an electrochemical reaction, while fuel is supplied to the fuel electrode and air to the air electrode. The fuel for use is, for example, hydrogen or methanol. The output voltage of such a fuel cell does not always agree well with the power-supply voltage for operation that the load device demands, and thus, used is a DC-DC converter that converts the output voltage of the fuel cell to the power-supply voltage for operation of the load device (see, for example, Japanese Patent Unexamined Publication No. 2006-501798 (FIG. 1A)).
FIG. 6 is a circuit diagram showing a power supply device employing the DC-DC converter described in the BACKGROUND OF THE INVENTION. The power supply device 101 shown in FIG. 6 has a fuel cell 102, a DC-DC converter 103, and a secondary battery 104, and the terminal voltage of the secondary battery 104 is outputted to a load 105. The DC-DC converter 103 is a voltage-boosting DC-DC converter that raises the output voltage of the fuel cell 102. The DC-DC converter 103 has a coil 106, a diode 107, and a switching element 108.
The cathode terminal of the fuel cell 102 is connected via the coil 106, the diode 107, and the secondary battery 104 to the anode terminal of the fuel cell 102. In addition, the switching element 108 is connected in parallel with the series circuit of the diode 107 and secondary battery 104. In the DC-DC converter 103 in such a configuration, there is generated power loss at a magnitude of the product of the forward voltage of the diode 107 and the output current during flow of the output current supplied to the load through the diode 107.
For example, even when a schottky barrier diode having a smaller forward voltage is used as the diode 107, the forward voltage thereof is at least 0.3 V. Accordingly, if the output current is for example 10 A, the power loss in the diode 107 is 10 A×0.3 V=3 W.
Recently, synchronous-rectification DC-DC converters, which use a small on-resistance power MOSFET (Metal Oxide Semiconductor Field Effect Transistor) replacing the diode 107, have been used more frequently. FIG. 7 is a circuit diagram showing a power supply device 110 employing such a synchronous-rectification DC-DC converter 112. The DC-DC converter 112 is different from the DC-DC converter 103 shown in FIG. 6, in that the diode 107 is replaced with a switching element 116. Also in the power supply device 110, a secondary battery 113 is used, replacing the secondary battery 104 in the power supply device 101. The fuel cell 102, which is less sensitive to fluctuation of the load current, absorbs the fluctuation of load current caused by charge/discharge of the secondary battery 113.
In such a case, when a power MOSFET (e.g., IRF7809, manufactured by International Rectifier) is used as the switching element 116, the on-resistance becomes 7 mΩ. If the output current is for example 10 A, the power loss then in the switching element 116 is 10 A×10 A×7 mΩ=0.7 W, which is lower than the power loss in the diode 107 of the DC-DC converter 103 described above.
In the DC-DC converter 112 in such a configuration, a control signal at a particular duty ratio is outputted from a switching controller 114 to a switching element 108 and an inverter 117, and the signal inverted by the inverter 117 is outputted to the switching element 116. In this way, the DC-DC converter 112 controls on/off duty by turning the switching elements 108 and 116 on and off alternately by the switching controller 114, and thus controls the output voltage of the fuel cell consistently.
The voltage multiplication ratio (Vo/Vi) of the DC-DC converter 112 is represented by the following Equation (1), wherein Vi denotes an output voltage of the fuel cell, or an input voltage of the DC-DC converter 112; Vo denotes an output voltage of the DC-DC converter 112; and D denotes an on-duty ratio of the switching element 108:Vo/Vi=1/(1−D)  (1).
Because the secondary battery 113 is connected to the output terminal of the DC-DC converter 112, the output voltage Vo in Equation (1) is equal to the output voltage of the secondary battery 113. The on-duty ratio D is so controlled that the Vi then reaches a preset value.
However, use of a synchronous-rectification DC-DC converter 112 for reduction of power loss results for example in decrease of the output power of the fuel cell 102 by insufficient supply of fuel, significant increase of the output impedance of the fuel cell 102, and thus, deterioration of the output current of the fuel cell 102, and then, the current flow through the switching element 116 in the reverse direction, differently from the diode 107, and the on-duty ratio D is so controlled that the output voltage of the fuel cell Vi becomes a preset value. In such a case, there is a concern about the current flow in the reverse direction from the secondary battery 113 via switching element 116 and coil 106 to the fuel cell 102 damaging the fuel cell 102.
As described above, in a power supply device employing a fuel cell, an energy storage element such as secondary battery or capacitor is connected to the output terminal of the DC-DC converter, because of the fuel cell's characteristic of low responsiveness to the fluctuation in load current. There remained still problems that, when such an energy storage element is installed in the power supply device employing a fuel cell, use of a small-power-loss synchronous-rectification DC-DC converter may cause current back flow from the energy storage element to the fuel cell, and that it may damage the fuel cell because the fuel cell is characteristically degraded by the current back flow.